1. Field of the Invention
The present invention relates to multi-shaft auger systems and processes for mixing soil with a chemical hardener in situ to form soil-cement columns, walls, piles, grids and monolithic blocks of overlapping columns. More particularly, the present invention is directed to improvements in auger shafts which permit more efficient penetration and improved mixing of the chemical hardener with the soil which forms the soil-cement columns, walls, piles, grids, and monolithic block of columns.
2. The Relevant Technology
For a number of years, multi-shaft auger machines have been used to construct soil-cement columns in the ground without having to excavate the soil by boring into the ground, injecting a chemical hardener and mixing the chemical hardener with the soil. These columns are sometimes referred to as "soilcrete" columns. Soilcrete is a term applied to a mixture of soil and a chemical hardener, which sets up as a solid mass, much like concrete. A "soilcrete" column is one of the most common products of in situ mixing of soil and chemical hardener, so it is used as a generic term to describe the hardened product of in situ soil mixing.
The chemical hardener is injected directly into the soil in situ, and mixed with the soil by an auger. The term "chemical hardener" includes any chemicals and agents that can be added and mixed with soil to cause chemical reactions resulting in the formation of soil-cement structural units. Examples of suitable chemicals and agents are: portland cement, lime, fly ash, kiln dust, cement-based hardeners, bitumen, resin, power plant residues, bentonite, salts, acids, sodium and calcium silicates, calcium aluminates, and sulfates. The chemical reactions include pozzolanic reaction (cementation), hydration, ion-exchange, polymerization, oxidation, and carbonation. The results of these chemical reactions include changes in the physical properties of soil such as strength and permeability and/or the change of chemical properties such as the reduction of the toxicity level in contaminated soil or sludge.
The chemical hardener is added in a slurry form. Therefore, the term "slurry" as used herein is defined as including chemical hardener. Cement slurry has also been called cement grout or cement milk in some of the previous techniques.
Upon hardening, the soil-cement columns possess some characteristics of lower strength concrete columns, but they are constructed without the expense and time-consuming process of removing and replacing the soil with concrete. In some cases, non-hardening soil-chemical or soil agent mixtures are also desirable.
Soil-cement columns have been arranged in a variety of patterns depending on the desired application. Soil-cement columns are used to improve the load bearing capacity of soft soils, such as sandy or soft clay soils. The columns are formed deep in the ground to help support surface construction on soft soils.
In other cases, the soil-cement columns have been overlapped to form boundary walls, excavation support walls, low to medium capacity soil-mixed caissons, and for the in situ fixation of contaminated soil or toxic wastes.
To produce soil-cement columns, a multi-shaft auger machine bores holes in the ground and simultaneously mixes the soil with a slurry or slurries of chemical hardener pumped from the surface through the auger shaft to the end of the auger. Multiple columns are prepared while the soil-cement mixture or soil-chemical mixture is still soft to form continuous walls of geometric patterns within the soil depending on the purpose of the soil-cement columns.
Because the soil is mixed in situ and because the soil-cement wall is formed in a single process, the construction period is shorter than for other construction methods. Obviously, the costs of forming soil-cement columns are less than traditional methods requiring excavation of the soil, constructing forms, and then pouring concrete into the forms in order to form the concrete pillars or walls. In addition, because the soil is not removed from the ground, there is comparatively less material produced in situ by such processes that must be disposed of during the course of construction.
FIG. 1 illustrates a conventional multi-shaft auger machine as the machine would appear in operation. Each shaft of the multi-shaft auger machine, is shown generically as shaft 20. The power for rotating the shaft is generated by a motor 26 and transferred to the upper end of each shaft 24 through a gearbox 22. This configuration is an example of a means for rotating the shafts by generating power and transferring the power to the shaft. Auger blades are securely affixed to the lower end of each shaft for boring downward through the soil to auger boreholes.
As the multi-shaft auger machine penetrates the soil, the soil is broken loose and a chemical hardener slurry is injected into the soil. The chemical hardener is be pumped from the surface through the auger shafts, which are hollow, to the lower ends of each shaft. The augers penetrate, break loose, and lift the soil so that it is mixed with the slurry by the action of intermittent soil mixing paddles 28 and intermittent auger blades 30 which are spaced throughout the length of the shaft. The horizontal and vertical mixing of the auger blades 30 and the soil mixing paddles 28 produces a column having a homogeneous mixture of the soil and the chemical hardener.
FIG. 2 illustrates the details of a prior art three-shaft auger machine. The three-shaft auger machine contains two outer shafts 31a and 31b and a center shaft 32 each having an upper end (not shown) and a lower end shown generally on the two outer shafts as 33a and 33b and on center shaft as 34. Outer auger blades 36a and 36b and center auger blades 38 penetrate undisturbed soil as the shafts rotate and propel the shafts downward to auger boreholes. The outer auger blades 36a and 36b and center auger blades 38 are securely affixed to lower ends 33a, 33b and 34 of outer shafts 31a and 31b and center shaft 32. The outer auger blades 36a and 36b are vertically offset from center auger blades 38. The outer auger blades 36a and 36b and center auger blade 38 each possess auger cutting edges 40 which cuts into the soil at the bottom of each borehole. Auger teeth 42 are preferably secured to the cutting edge of the first and second auger blades in order to assist in soil penetration in clay or rocky soils.
Generally, each shaft on a multi-shaft auger machine with three or more shafts rotates in a direction opposite the rotation of adjacent shafts. As shown in FIG. 2, auger blade 38 attached to the lower end 34 of center shaft 32 has a spiral configuration opposite the auger blades attached to the lower ends 33a and 33b of outer shafts 31a and 31b. Thus, if center shaft 32 rotated in a clockwise direction, outer shafts 31a and 31b would rotate in a counter-clockwise direction.
During operation the auger machine starts to penetrates downward through the soil. The process of penetrating downward is often referred to as an auguring stroke. As the auger blades move down, the injection of slurry through the auger shaft is initiated. As the slurry exits the auger shaft, it is mixed with the soil by the auger blades and mixing paddles along the length of each auger.
The resulting soil and slurry mixture is referred to as a "column set" or "borehole". The use of the term "borehole" does not necessarily mean that soil is removed to create a hole. Although some soil is deposited on the surface due to expansion of the soil as it is broken loose and mixed, the majority of the soil remains below the surface as it is mixed. Moreover, use of the term "column set" may refer to a single in situ column set formation or it may generically refer to wall formations or continuous large-area soil formations. Such columns are sometimes referred to as "piles". The column set may be extended to form a grid or a monolithic block of overlapping columns.
The mixing ratio of the slurry to the soil is determined on the basis of the soil conditions which are determined and reported prior to commencing the boring of the columns. The soil-slurry mixing ratio is not decided on the basis of the strength conditions of the continuous wall alone, but such factors as the soil type and condition, and the state of ground water are also taken into consideration in order to obtain a mixing ratio which will result in a substantially homogenous wall which has the desired strength and permeability characteristics. In some cases, special chemical slurries are mixed with in situ soil to stabilize and/or solidify various pollutants in the soil--a procedure named in situ solidification and stabilization or in situ fixation.
Slurry is continuously pumped through the center of the auger shaft and mixed with the soil as the augers penetrate and are then withdrawn from the borehole. In a typical process about 60 percent to 80 percent of the slurry is injected as the augers penetrate downward and the remainder is injected as the augers are withdrawn. According to this method, the mixing process is repeated as the augers are withdrawn from the borehole. Auger speed and slurry output quantities are also set to meet the soil conditions of the site and the purposes of soil mixing work.
The resulting mixing of soil and chemical hardener is sometimes referred to as "soilcrete" because the hardener mixture often possesses some physical properties similar to concrete. Nevertheless, use of the term "soilcrete" does not mean that soil is mixed with concrete or that the chemical hardener always contains cement. If cement slurry is used, the preferred term to describe the hardened mixture is soil-cement.
Due to the tremendous forces required to push the shaft downward and to turn the augers and the shaft, as well as the tendency of the multiple shafts to diverge due to varying soil conditions encountered by each shaft, a lateral support structure is provided to prevent diversion of the auger shafts out of a parallel configuration while concomitantly allowing the shafts to rotate. The lateral support structure, generally illustrated at 50, is located about each respective shaft such that the lateral support structure does not rotate as each respective shaft rotates in the soil. As the lateral support structure 50 serves to maintain the auger shafts in a parallel configuration, the lateral support structure must be located fairly near to the lower ends of the shafts where the impact of rocks and varying soil textures has the most effect on the shafts.
The lateral support structure 50 typically takes the form of nonrotating bands 52 surrounding each shaft, stabilizing bars 54 securely connecting the nonrotating bands to maintain proper shaft alignment and clamps 56a and 56b securely attached to the nonrotating bands opposite the stabilizer bars 54 to provide additional support.
The nonrotating bands 52 surround the shafts in an area between upper cylindrical collars 58 and lower cylindrical collars 60, the cylindrical collars are formed around the periphery of each shaft. The use of bearings and the configuration of the nonrotating bands and the upper and lower cylindrical collars allow the shafts to rotate within the nonrotating bands 52 while the nonrotating bands remain stationary.
As the augers penetrate new soil, the soil is loosened and forced past the lateral support structure 50 by the action of the rotating auger blades pushing more soil up from below. After passing the lateral support structure, the soil is remixed with mixing paddles attached to the shaft above the lateral support structure.
This auger system works well in sandy or porous soils, however, problems are encountered when auguring in cohesive soils such as clay or silt and with slurries having a low water content. When the auger blades located at the end of each shaft encounter such soils and slurries, the augers shear and fragment the soil only to have the soil reaggregate before passing the lateral support structures which support the shafts. This reaggregation or reagglomeration of soil can form a cylindrical plug which impedes optimal mixing as the plug rotates with the shaft. The natural tendency of cohesive soils such as clay or silt to coalesce is further exacerbated by the injection of the slurry, particularly slurries with a low water content, as the slurry increases the tendency of clay and silt to prematurely reagglomerate.
When sufficient pressure is exerted on the plug by the action of the augers on new soil being forced up from below, the clay plug is forced around the lateral support structure into the area of the borehole above the lateral support structure. Once the cylindrical plug reaches the mixing blades located above the lateral support structure, the cylindrical plug must once again be sheared, fragmented and thoroughly mixed with the slurry. The cylindrical plug reformed beneath the lateral support structure must undergo essentially the same process above the lateral support structure as the process the soil was subjected to below the structure. The mixing blades and paddles located on the shafts above the lateral support structure must not only mix the soil but also reshear and refragment it. The additional shearing and fragmenting results in a reduced rate of progress by the auger machine through the soil. Further, there is significantly less homogenous mixing of the soil with the slurry which results in columns having decreased strength.
The need for shearing and fragmenting the soil and slurry above the support structure requires the use of much more energy which impedes optimal mixing. This energy must be deducted from the total energy available for penetrating new soil layers. This reduction in available energy results in less efficient boring, both in rate of progress through the soil and in the thoroughness of mixing of the soil with the slurry.
From the foregoing, it will be appreciated that what is needed in the art is a multi-shaft auger system for mixing soil with a chemical hardener in situ, which provides for a more homogenous mixture of a chemical hardener slurry and soils when utilized with soils tending to coalesce and with slurries with a low water content.
It would be another advancement in the art to provide a multi-shaft auger system for mixing soil with a chemical hardener in situ, which improves the efficiency of systems utilized in soils tending to coalesce and with slurries with a low water content.